Based on the way The Avengers ends, I think the most likely villain for Avengers 2 is

Spoiler!!! Click to Read!:

Thanos

, although there is also the potential for him to appear as the villain of Thor 2 before then, which would make him the second Marvel villain to appear in both a Thor movie and an Avengers movie (the first, of course, being Loki).

Speaking of which is it confirmed that Thor 2,Iron man 3,and Captain America 2,next Hulk,etc will all or some of them be made before the next avengers film?
I'm guessing it's too early to say for sure but with the money The Avengers has been bringing in i'm wondering if they'll just make the sequel before anything else?